1. Field of the Invention
This invention relates generally to a method for controlling a vehicle powertrain during a vehicle launch event using an engine and a stepped ratio transmission.
2. Description of the Prior Art
A powershift transmission is a geared mechanism having two input clutches, which alternately connect a power source, such as an engine to two transmission input shafts.
The transmission produces multiple gear ratios in forward drive and reverse drive though operation of gearing arranged in a dual layshaft configuration between the transmission input and its output. One input clutch transmits torque between the input and a first layshaft associated principally with even-numbered gears; the other input clutch transmits torque between the transmission input and a second layshaft associated principally with odd-numbered gears. The transmission produces gear ratio changes by alternately engaging a first input clutch and running in a current gear, disengaging the second input clutch, preparing a power path in the gearing for operation in the target gear, disengaging the first clutch, engaging the second clutch and preparing another power path in the gearing for operation in the next gear.
Because a dual clutch transmission has no torque converter to provide damping, such transmissions begin and end each gear shift with the holding clutch, i.e., the input clutch through which engine torque is transmitted to the transmission input shaft for the target gear, slipping. To provide acceptable shift quality, it is necessary to maintain the correct slip without excess flare and without locking the holding clutch. At the same time, the input clutches must maintain sufficient torque at the output shaft to provide consistent acceleration before, during and after the shift.
The magnitude slip across the input clutches must be closely controlled.
A vehicle equipped with a dual clutch powershift transmission can be stopped but the gear in which the transmission is operating may not be its launch gear, i.e., first gear. When the driver applies the accelerator pedal to launch the vehicle, an unresponsive undesirable performance feel can be produced.